1. Field of the Invention
The present patent specification relates to a sheet conveying device and an image forming apparatus incorporating the sheet conveying device.
2. Description of the Related Art
As a method of separating and conveying stacked sheets, such as documents and recording sheets, a separating and conveying method using frictional force and a separating and conveying method based on air suction have been used. The separating and conveying method using frictional force typically uses, for example, a rubber feeding roller, and as a result the frictional force changes over time due to abrasion and other factors, such that the conveying performance is degraded. Further, when sheets non-uniform (i.e., varying) coefficient of friction or sheets having different coefficients of friction are separated and conveyed in the same separating and conveying operation, a feeding failure occurs in some cases, which includes simultaneous multiple feeding of a plurality of sheets and a failure to separate sheets. Further, in some cases, the sheets are stained due to a configuration that separates the sheets by applying pressure thereto in the sheet conveying operation.
Meanwhile, the separating and conveying method using air suction is a non-frictional separation method not relying on the coefficient of friction of rollers and sheets. The method, however, uses an air suction blower and an air duct. Thus, the sheet conveying device is increased in size, and air suction sound itself is noise. Therefore, the device is not suitable for use in an office environment.
In view of the above, an electrostatic method as one type of non-frictional separation method has been proposed, which generates an electric field in a dielectric belt and brings the dielectric belt into contact with a sheet to simultaneously attract the sheet and separate the sheet from other sheets. According to the electrostatic method, an attraction belt wound around a plurality of rollers is supplied with an alternating charge and translated relative to a sheet stack such that the attraction belt approaches or comes into contact with the sheet stack to attract the uppermost sheet of the sheet stack. Thereafter, the attraction belt is moved in a direction separating from the sheet stack to separate the uppermost sheet of the sheet stack from the sheet stack. The electrostatic method is advantageous in preventing, for example, abrasion, damage to the sheet, and noise, and reducing the overall size of the device.
Despite its success, several problems remain with the electrostatic method as conventionally implemented, as is now described.
FIG. 1 illustrates a background example of an attractive separation unit 201a, which includes an attraction belt 200a wound around a plurality of rollers, and which is translated to attract and separate the uppermost sheet S1 of a sheet stack from the sheet stack. In the configuration of FIG. 1, the uppermost sheet S1 of the sheet stack is attracted to the attraction belt 200a, and thereafter the attraction belt 200a is moved in a direction separating from the sheet stack. In this state, the electrostatic attraction force acting between the uppermost sheet S1 and the attraction belt 200a is stronger than the weight of the uppermost sheet S1. Therefore, the uppermost sheet S1 is attracted to the attraction belt 200a. Meanwhile, the distance between the second sheet and the attraction belt 200a is greater than the distance between the uppermost sheet S1 and the attraction belt 200a. Thus, the electrostatic attraction force of the second sheet is weaker than the electrostatic attraction force of the uppermost sheet S1, and falls below the weight of the second sheet. As a result, the second sheet remains on the sheet stack, and is separated from the uppermost sheet S1.
However, if the thickness of the sheets is reduced, the distance between the second sheet and the attraction belt 200a is reduced, and the weight of the sheet is also reduced. Therefore, the electrostatic attraction force of the second sheet is stronger than the weight of the second sheet. In some cases, therefore, the uppermost sheet S1 and the second sheet fail to separate from each other.
FIG. 2 illustrates another background example of an attractive separation unit 201b, which includes a dielectric attraction belt 200b stretched taut by rollers 202b and 203b located downstream and upstream, respectively, in the sheet conveying direction (hereinafter referred to as the downstream-side roller 202b and the upstream-side roller 203b, respectively). The attractive separation unit 201b is swung around the upstream-side roller 203b as the center of rotation to attract and separate the uppermost sheet S1 of the sheet stack from the sheet stack. In the configuration of FIG. 2, the uppermost sheet S1 of the sheet stack is attracted to the attraction belt 200b, and thereafter the attractive separation unit 201b is swung around the upstream-side roller 203b as the center of rotation to separate the downstream-side roller 202b from the sheet stack. In this configuration, when a sheet electrostatically attracted to the attraction belt 200b is going to move together with the attraction belt 200b, the sheet is bent at a portion of the attraction belt 200b in contact with the upstream-side roller 203b as a fulcrum, and restoring force acts on the sheet. The attraction force of the uppermost sheet S1 toward the attraction belt 200b is more than the restoring force of the uppermost sheet S1. Thus, the uppermost sheet S1 moves together with the attraction belt 200b. Meanwhile, the distance between the second sheet and the attraction belt 200b is more than the distance between the uppermost sheet S1 and the attraction belt 200b, and the attraction force of the second sheet toward the attraction belt 200b is less than the restoring force of the second sheet. As a result, the second sheet separates from the attraction belt 200b. With this use of the restoring force (i.e., rigidity) of sheets, favorable separation performance is obtained.
In the configuration of FIG. 2, however, the upstream-side roller 203b is desired to be separated from the sheet stack in the conveyance of the uppermost sheet S1 separated and attracted to the attraction belt 200b. This is because, in a configuration which moves the upstream-side roller 203b while in contact with the sheet stack, after the rear end of the uppermost sheet S1 passes under the upstream-side roller 203b, the second sheet receives the conveying force of the upstream-side roller 203b and thus moves in the sheet conveying direction. Therefore, the configuration of FIG. 2 includes a device for lifting the attractive separation unit 201b in addition to a device for swinging the attractive separation unit 201b, which adds to the complexity, size, and cost of the device.
FIGS. 3A to 3C illustrate yet another background example of a sheet conveying device 220. In an adhesive separation unit 201c of the sheet conveying device 220, an adhesion belt 200c is stretched taut by a driven roller 202c, a drive roller 203c, and a tension roller 204. Further, the adhesive separation unit 201c includes a charging roller 205 that serves as a charging device which charges a surface of the adhesion belt 200c and a roller 206 which comes into contact with the uppermost sheet S1 and rotates together with the uppermost sheet S1. The driven roller 202c, the drive roller 203c, the tension roller 204, the charging roller 205, and the roller 206 are rotatably supported by a not-illustrated side plate of the adhesive separation unit 201c. The side plate is configured to be rotatable around a rotary shaft of the drive roller 203c. 
In the conveyance of the uppermost sheet S1 of a sheet stack S stacked on a sheet feeding tray 210, the sheet feeding tray 210 is lifted to bring the uppermost sheet S1 into contact with the roller 206. Then, the attraction belt 200c is rotated, and the surface thereof is applied with an alternating charge by the charging roller 205. Then, the attractive separation unit 201c is rotated in the counterclockwise direction in the drawings around the rotary shaft of the drive roller 203. Thereby, an area of the attraction belt 200c located between and stretched by the driven roller 202c and the tension roller 204 is brought into contact with the uppermost sheet S1 to electrostatically attract the uppermost sheet S1 to the attraction belt 200c (see FIG. 3B). Then, the attractive separation unit 201c is rotated in the clockwise direction. Thereby, a sheet electrostatically attracted to the attraction belt 200c is going to move, together with the attraction belt 200c. In this state, the sheet is bent at a portion thereof in contact with the roller 206 as a fulcrum, and restoring force acts on the sheet. The attraction force of the uppermost sheet S1 toward the attraction belt 200c is more than the restoring force of the uppermost sheet S1. Thus, the uppermost sheet S1 moves together with the attraction belt 200c. Meanwhile, the distance between the second sheet and the attraction belt 200c is more than the distance between the uppermost sheet S1 and the attraction belt 200c, and the attraction force of the second sheet toward the attraction belt 200c is less than the restoring force of the second sheet. Thus, the second sheet separates from the attraction belt 200c (see FIG. 3C). Then, the attraction belt 200c is rotated to convey the uppermost sheet S1 attracted thereto toward a conveying roller, pair.
In the sheet conveying device 220, the center of swing of the attractive separation unit 201c is set to a position upstream in the sheet conveying direction of the area of the attraction belt 200c coming into contact with the uppermost sheet S1. Thus, the attraction belt 200c is separated from the sheet stack S simply by the swing of the attractive separation unit 201c. Accordingly, there is no need to provide a device for lifting the attractive separation unit 201c. Further, with the roller 206 brought into contact with a sheet, favorable separation performance is obtained. Further, the roller 206 is configured to rotate together with a sheet, and does not rotate after the rear end of the uppermost sheet S1 passes under the roller 206. Accordingly, the second sheet does not receive the conveying force.
The sheet conveying device 220, however, includes three rollers, i.e., the driven roller 202c, the drive roller 203c, and the tension roller 204 for keeping the attraction belt 200c taut, and also includes the roller 206. This configuration, therefore, increases the number of components and the cost of the sheet conveying device 220.